1. Field of the Invention
The present invention relates to a multiplexer including an elastic wave filter, a transmission device, and a reception device.
2. Description of the Related Art
In recent years, there has been a demand for cellular phones to be able to handle multiple frequency bands and multiple wireless formats with a single terminal, i.e., to have multi-band and multi-mode capabilities. To this end, a multiplexer is disposed immediately beneath a single antenna, to demultiplex high-frequency signals having multiple wireless carrier frequencies. An elastic wave filter having low loss in the passband, and steep passband characteristics around the passband, is used as the multiple passband filters defining the multiplexer.
International Publication No. 2016/208670 discloses a surface acoustic wave device (SAW duplexer) having a configuration in which multiple surface acoustic wave filters are connected. Specifically, an inductance element is connected in series between a connection path to an antenna terminal of a reception-side surface acoustic wave filter and a transmission-side surface acoustic wave filter, and an antenna element, to enable impedance matching between the antenna element and antenna terminal. This inductance element enables complex impedance, when viewing the surface acoustic wave filter from the antenna terminal to which multiple surface acoustic wave filters that have capacitance are connected, to be brought closer to the characteristic impedance. Thus, International Publication No. 2016/208670 describes that deterioration of insertion loss is able to be prevented.
Regarding connection of a transmission terminal of a transmission-side surface acoustic wave filter to a power amplifier (PA) and a reception terminal of a reception-side surface acoustic wave filter to a low-noise amplifier (LNA), the characteristic impedance of the transmission terminal of the transmission-side surface acoustic wave filter and the reception terminal of the reception-side surface acoustic wave filter is sometimes designed to match the respective PA and LNA in recent years, in order to reduce the number of matching elements to perform impedance matching. However, the characteristic impedance of the antenna-side terminal of the transmission-side surface acoustic wave filter and the reception-side surface acoustic wave filter is 50Ω, and therefore, the characteristic impedance may be different between the transmission terminal or the reception terminal side of the surface acoustic wave filter and the antenna terminal side. The surface acoustic wave device and impedance matching method described in International Publication No. 2016/208670 is not able to sufficiently match impedance for each terminal in this case, and there is a problem that insertion loss increases.